[TowerTalk] Re: 20M Yagi Interaction with nearby objects

[Pete Smith]

At 03:25 PM 10/20/03 -0600, Ve6wz_Steve wrote:
>I think sometimes we go "overboard" with this interaction issue.
>
>Using any modeling program, start with a dipole and then introduce
>another wire and TRY to make a pattern.
>
>Only when the second wire is close to resonance, and at the right
>spacing are the currents correct to develop a pattern.  This new wire
>needs to be accurate within inches in length to make a pattern. After
>all, this is why it can be so difficult and time consuming to adjust a
>Yagi model (or in the real world on a tower) for optimum F/B.  That
>reflector (or director) has to be "just right" or it won't do a thing.

This certainly isn't my real-world experience.  A loaded 40-meter yagi 
element is resonant who-knows-where on its third harmonic (probably not 
VERY close to the 15-meter ham band), and yet instances of interaction with 
15-meter yagis are more common than not.  My 40-meter yagi was a dog until 
I replaced the top (grounded) guys on my tower with Phillystran, at which 
point both pattern and gain improved noticeably.  The conventional wisdom 
about guy interaction was that you needed to avoid resonant lengths, but 
there is growing empirical evidence that this isn't true.  Serendipitously, 
my tower acts as a fairly effective reflector for my 80-meter dipole array, 
even though it is nowhere near a resonant length that you can discern from 
measurement or modeling.  If it is removed from the array, pattern and gain 
suffer.


73, Pete N4ZR
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